The invention relates to a bi-polar transistor and a procedure, for its manufacture.
The implementation of epitaxially manufactured silicon-germanium hetero-bi-polar transistors (SiGe HBT) and the resulting cost-reducing simplification of the technological processes have lately provided a new impetus for a further development of Si bi-polar transistors. In this respect, the combination of an epitaxially produced base with the process-simplifying possibilities of the single polysilicon technology offers an attractive direction of development.
In comparison with conventional base profiles produced by implantation or diffusion, silicon-germanium base layers made by epitaxy allow producing, simultaneously, smaller base widths and base layer resistance without unusable small current gains or high leakage currents. The technology allows implementation of a concentration of the active doping agent of up to 1xc3x971020 cmxe2x88x923, as is describedxe2x80x94for examplexe2x80x94in A. Schxc3xcppen, A. Gruhle, U. Erben, H. Kibbel und U. Kxc3x6nig: 90 GHz fmax SiGe-HBTs, DRC 94, page IIA-2, 1994. However, in order to prevent leakage currents due to tunnel processes, a low-doped region is required between the high-concentration zones of the emitter and the base. As a matter of fact, if the base doping exceeds the value of 5xc3x971018 cmxe2x88x923, and if the high concentration of the emitter reaches down to the basexe2x80x94as is usual with implanted base profilesxe2x80x94the consequence is the existence of unacceptably high tunnel currents. As opposed to implanted base profiles, the application of epitaxy allows, simultaneously and without any problems, the production of narrow base profiles and a low-doped region (cap layer).
FIG. 1 illustrates the emitter zone of a SiGe HBT. This transistor design reflects typical characteristics of a single poly-silicon process. An SiGe base 12 and subsequently a cap layer 13 were deposited over a monocrystal collector zone 11. FIG. 1 does not show a lateral insulation of the transistor zone. If semiconductor material grows both on the monocrystal substrate 11 and on the insulator zonexe2x80x94not shown in the picturexe2x80x94(i.e., differential epitaxy), it is possible to utilize the grown semiconductor layers as a connection between a contact on the insulation zone and the inner transistor. Such a connection should be, designed with as low impedance as possible. This is why it would be advantageous if the epitaxial layer thickness could be set up independently from the base width. A poly-silicon or an xcex1-silicon layer 15 is deposited on an insulation layer 14, in which emitter windows were etched by means of a wet-chemical etching process. During the deposition or subsequently, the xcex1-silicon layer 15 obtainsxe2x80x94by implantationxe2x80x94a doping of the emitter""s conductivity type and serves as diffusion source for the emitter doping 16 in the monocrystal substrate. Insulator layer 14 is applied in order to prevent damage to cap layer 13 during the structuring of the polycrystal xcex1-silicon layer 15 performed later. In the overlapping region 17xe2x80x94a zone between the edge of the emitter window and the outer delimitation of the structured poly-silicon or xcex1-silicon layer 15, a layer, sequence arises consisting of semiconductor material, insulator material and semiconductor material. Depending on the doping of the cap layer 13, the interfacial charges and the recombination properties of the surface as well as on the operation conditions of the transistor, this design can causexe2x80x94analogous to a MOS capacityxe2x80x94an enhancement but also a depletion of mobile charge carriers on the surface of the cap layer 13. With a forward-current base-emitter diode, this can affect both the ideal nature of the base current and the low-frequency noise properties. Under certain circumstances, generation currents and breakdown voltage in the non-conducting direction can be affected. The condition thatxe2x80x94due to the tunnel (currents) dangerxe2x80x94the doping agent concentration should not exceed the value of 5xc3x971018 cmxe2x88x923 leads to the question, by means of which procedure this zone should be suitably doped. The following text discusses the variants for SiGe HBT so far known: homogeneous n-doping or p-doping near the tunnel limit or quasi undoped zones (i-zones). A. Chantre, M. Marty, J. L. Regolini, M. Mouis, J. de Pontcharra, D. Dutrtre, C. Morin, D. Gloria, S. Jouan, R. Pantel, M. Laurens and A. Monroy: A high performance low complexity SiGe HBT for BiCMOS integration, BCTM ""98, 1998, pages 93-96 uses a p-doping of about 5xc3x971018 cmxe2x88x923. This results in a decisive disadvantage in that the thickness of, the cap layer must be set up within a tolerance range of a few nanometers from the penetration depth of the doping agent diffusing from the poly-silicon emitter layer. Greater cap layer thickness values (which would be advantageous for a low-impedance connection between the inner base and a connector in the insulation zone) are not possible since it would negatively affect the effect of the germanium profile. A. Gruhle, C. Mxc3xa4hner: Low l/f noise SiGe HBTs with application to low phase noise microwave oscillators, Electronics Letters, Vol. 33, No. 24, 1997, pages 2050-2052 uses a cap layer 100 nm thick with an n-concentration of 1-2xc3x971018 cmxe2x88x923. EP-A-0 795 899 indicates similar conditions, where preferably a cap layer of a thickness of 70 nm with a n-doping concentration of 2xc3x971018 cmxe2x88x923 is used. Although this variant eliminates the problem of the thickness tolerance, and avoids the danger of tunnel currents by reducing the doping agent concentration in the cap layer, it still does not take full advantage of the possibilities of reducing the base-emitter capacity.
This disadvantage can be eliminated by not doping the cap layer as is described, for example, in B. Heinemann, F. Herzel and U. Zillmann: Influence of low doped emitter and collector regions on high-frequency performance SiGe-base HBTs, Solid-St. Electron, 1995, Volume 38(6), pages 1183-1189. However, it can easily lead to a depletion of the aforementioned overlapping region 17. These connections are explained in further text by means of a two-dimension design element simulation.
FIG. 2 shows the simplified transistor design used in the simulation. The electrical effect of the oxide semiconductor surface in the overlapping region is modeled by means of a positive surface charge density of 1xc3x971011 cmxe2x88x922 and a surface recombination speed of 1000 cm/s. FIG. 3 illustrates vertical profiles along a section line horizontal to the overlapping region. The profiles show three doping variants in the cap layer 13 and a p-doped SiGe base 12 identical for all three cases. The following cap doping cases are compared: a quasi undoped cap layer 13 (profile i) and two homogeneous n-dopings (profile n1 with 1xc3x971018 cmxe2x88x923 and profile n2 with 2xc3x971017 cmxe2x88x923). FIG. 4 shows the transition frequency as a function of the collector current for various doping variants. Especially with small collector currents, an increase in transition frequency with a falling doping level in the cap layer 13 can be noticed. While profile i provides relatively best transition frequencies, it has, however, the disadvantage that the ideal nature of the base current (FIG. 5) is noticeably affected in comparison with the other profiles.
The task of this invention is to indicate a bi-polar transistor and a procedure for its manufacture that eliminates the described disadvantages of conventional arrangements, in order to achieve especially minimal base-emitter capacities and best high-frequency properties without noticeably affecting the static properties of the bi-polar transistor with a low-doped cap layerxe2x80x94above all the ideal nature of the base current and low-frequency noisexe2x80x94and without increasing the process complexity.
This invention fulfills this task by introducing a special doping profile into an epitaxially produced cap layer (cap doping). This doping profile allows achieving a minimal base-emitter capacity and best high-frequency properties, but also restricts the effect of a generation-active and recombination-active interface between the cap layer and the insulator in the overlapping poly-silicon region in the interesting function range of the transistor, and improves the ideal nature of the base current.
Decisive for the good high-frequency properties is the base-side section in the cap layer of a preferable thickness between 20 nm and 70 nm with low-concentration doping, preferably less then 5xc3x971016 cmxe2x88x923.
On the emitter side, the cap layer is doped more highly. If the doping agent is of a conductivity type like the base layer, the doping agent concentration applied in the cap layer is preferably less than 5xc3x971018 cmxe2x88x923 in order to prevent tunnel currents.
The cap doping profile is preferably introduced by implantation in situ during the epitaxy procedure.
The characteristics of this invention are clear from the claims and also from the description and the drawings, where each characteristicxe2x80x94either individually or several characteristics in the form of sub-combinationsxe2x80x94represent patentable designs, for which protection is demanded herein. Design examples are illustrated in the drawings and are explained in more detail in further text.